The present invention relates generally to methods for removing carbon dioxide from a gaseous mixture and more specifically to methods for removing carbon dioxide from air using lithium-exchanged faujasites.
Zeolite adsorbents for carbon dioxide removal from air have been a critical element of both the Skylab Regenerative Carbon Dioxide Removal System (RCRS) and the International Space Station (ISS) Carbon Dioxide Removal Assembly (CDRA). These adsorbents selectively separate carbon dioxide from air using either pressure swing (Skylab) or combined pressure swing/thermal swing (ISS) methodologies. The isotherm curves describing carbon dioxide uptake at various partial pressures are key determiners of the overall size and weight of an adsorbent-based system. The carbon dioxide adsorbent utilized in the ISS CDRA is a modified 5A zeolite first developed by AlliedSignal (now Honeywell) in the 1980's.
The size and configuration of the desiccant and carbon dioxide adsorbent beds are selected to optimize the system's performance. With respect to system performance, the key characteristics include carbon dioxide removal rate as a partial pressure function, system mass, and system power consumption. These key system requirements are utilized to develop the desiccant and adsorbent bed cycle times, process air flow rates, and regeneration temperatures.
The capacity of the adsorbent at the required carbon dioxide partial pressure is a key design parameter that determines the configuration and size of the beds. ASRT 5A, an improved version of the basic 5A zeolite, was developed for space applications. Its key advantage lies in its higher capacity at similar carbon dioxide partial pressures. In any aerospace application, component size and weight are crucial factors that determine the success of the technology. Therefore, any system that may be reduced in size or weight, especially in aerospace applications, is a significant achievement. Currently, carbon dioxide removal in the International Space Station uses a modified 5A sodium zeolite as the carbon dioxide adsorbent. Newer space missions, however, will require even higher performance, while minimizing the size, weight, power consumption, and waste heat emission.
As can be seen, there is a need for a high-performance carbon dioxide adsorbent with a large capacity for carbon dioxide during the adsorption portion, but also, a low capacity for carbon dioxide during regeneration. Such an adsorbent should minimize the size, weight, power consumption and waste heat emissions of a carbon dioxide removal system in which it is used.